Optical waveform for use in a DWDM optical network and systems for generating and processing same

ABSTRACT

An optical signal occupying one or more wavelengths. An optical data signal on each wavelength is modulated with a respective overhead (dither) signal, resulting in a respective dithered optical signal. The amplitude of a particular overhead signal used to modulate the corresponding optical data signal is chosen so that the RMS value of the overhead signal in the dithered optical signal is proportional to the average intensity of the optical data signal. The instantaneous frequency of each overhead signal is time-varying and each possible frequency belongs to a distinct set of frequencies which are all harmonically related to a fundamental frequency. The distinctness of each set of frequencies allows each overhead signal to be uniquely isolated from an aggregate overhead signal. The harmonic relationship among the frequencies allows improved accuracy of RMS detection at a receiver as well as reduced computational complexity, as each possible frequency for each overhead signal can be made to fall at the center of one of the frequency bins of a single FFT of reasonable size performed at a receiver. Methods and systems for generating and detecting such signals are disclosed.

This application is a division of U.S. patent Ser. No. 09/539,706 filedMar. 31, 2000.

FIELD OF THE INVENTION

The present invention relates generally to dense wavelength divisionmultiplexed (DWDM) optical networks and, in particular, to opticalwaveforms used in such networks and to systems used for generating anddecoding such waveforms.

BACKGROUND OF THE INVENTION

In a dense wavelength division multiplexed (DWDM) optical network, anaggregate optical signal travelling on a single optical fiber occupiesmultiple wavelengths, each of which carries a respective high-speedoptical data signal. Although they may travel together along multiplespans throughout the network, the individual optical data signals carryinformation that is independent and is associated with its own sourceand destination network elements.

It is often desirable for a downstream network element in a DWDM networkto have knowledge of the average intensity of the optical data signaloccupying each wavelength. This is to allow the network element toinitiate control functions such as amplifier gain adjustment andprotection switching in a timely fashion. In order to keep costs to aminimum, it is desirable for the downstream network element to haveaccess to the intensity information at multiple wavelengths withouthaving to resort to wavelength demultiplexing and conversion of multiple(possibly hundreds of) optical signals into electrical form.

A solution to this problem for low density wavelength divisionmultiplexed optical networks is described in U.S. Pat. No. 5,513,029 toRoberts et al., issued on Apr. 30, 1996, assigned to the assignee of thepresent invention and hereby incorporated by reference herein in itsentirety.

In accordance with U.S. Pat. No. 5,513,029, a transmitter firstmodulates the optical data signal on each wavelength with a respectivedither signal of a certain amplitude. The dither signals are describedas being pseudo-random noise (PN) sequences which are distinct for eachwavelength. The transmitter then measures the modulation depth of eachindividual resultant signal after modulation with the correspondingdither signal and adjusts the amplitude of the dither signal until aknown “modulation depth” is achieved. (The “modulation depth” iscommonly defined as the root-mean-square (RMS) value of the dithersignal divided by the average intensity of the optical data signal beingmodulated.)

Thus, a receiver located at a downstream network element can estimatethe average intensity of the optical data signal at multiple wavelengthsof an incoming optically multiplexed signal by: (a) tapping a portion ofthe multiplexed signal using an inexpensive optical coupler; (b)converting the tapped optical signal to electrical form; (c) low-passfiltering the electrical signal to provide an aggregate dither signal;(d) separating the dither signals by matching them with known PNsequences; (e) measuring the RMS value of each dither signal; and (f)dividing the RMS value of each dither signal by the known modulationdepth.

Unfortunately, the system disclosed in U.S. Pat. No. 5,513,029 hasseveral drawbacks, which either cause the system to have poorperformance or make the system prohibitively complex to implement, orboth. This is especially true in the context of a dense WDM network,where there may be hundreds of optical carriers sharing a relativelynarrow portion of the electromagnetic spectrum.

For example, in order to achieve good performance at the receiver, theRMS value of each dither signal present in the aggregate dither signalmust be determined to a high degree of accuracy. Of course, this canonly occur if the receiver accurately detects the presence of eachdither signal in the aggregate dither signal. If the individual dithersignals are PN sequences (as per U.S. Pat. No. 5,513,029), then in orderto achieve the requisite degree of accuracy, the receiver must samplethe aggregate dither signal at the center of the “chip period” of eachPN sequence.

However, because the dither signals are generated on an independentbasis, possibly by sources located at different parts of the network,there will generally be phase offsets among the dither signals containedin the aggregate dither signal. Therefore, in order to compensate forsuch differences in phase, the sampling rate used at the receiver mustbe much higher than the chip rate. It is therefore apparent that thereexists a trade-off between sampling precision (leading to accurate RMSestimates and hence accurate estimates of average intensity) andcomputational simplicity.

The problem is no less severe if the dither signals used for differentoptical wavelengths occupy different frequencies rather than differentPN sequences. In this case, the step of distinguishing the variousdither signals would involve passing the aggregate dither signal througha parallel array of filters, one for each wavelength. As the number ofwavelengths (and filters) increases, the bandwidth of each filter willhave to be decreased in order to allow only the desired dither signal topass through to the RMS detection stage, with the effect ofdeleteriously increasing the filter design complexity.

Moreover, it has been observed that framing of an optical data signal at8 kHz causes the appearance of spurious frequency lines at multiples of8 kHz in the frequency spectrum of the optical data signal. If suchlines fall within the bandwidth of one or more of the filters, then itis apparent that the RMS value estimated as a function of each suchfilter's output will be biased. In a more general case, e.g., where theoptical data signal is asynchronous, it may be impossible to predict thelocation of spurious frequency contamination, with the end result beingthe production of randomly biased RMS estimates.

Clearly, it would be a huge advantage to provide a system which iscapable of estimating the average intensity of an optical signal at eachof one or more wavelengths with reasonable accuracy and lowcomputational complexity.

SUMMARY OF THE INVENTION

The invention is directed to providing a method and system forestimating the average intensity of an optical signal at each of one ormore wavelengths with sufficiently high accuracy and sufficiently lowcomputational complexity.

In the present invention, the optical data signal on each wavelength ismodulated with a respective overhead (dither) signal, resulting in arespective dithered optical signal. The amplitude of a particularoverhead signal used to modulate the corresponding optical data signalis chosen so that the RMS value of the overhead signal in the ditheredoptical signal is proportional to the average intensity of the opticaldata signal. The instantaneous frequency of each overhead signal istime-varying and each possible frequency belongs to a distinct set offrequencies which are all harmonically related to a fundamentalfrequency.

The distinctness of each set of frequencies allows each overhead signalto be uniquely isolated from an aggregate overhead signal. In this way,one avoids having to perform optical demultiplexing of the individualdithered optical signals at a receiver in order to extract theindividual overhead signals. Furthermore, the harmonic relationshipamong the frequencies allows improved accuracy of RMS detection at areceiver as well as reduced computational complexity, as each possiblefrequency for each overhead signal can be made to fall at the center ofone of the frequency bins of a single FFT of reasonable size performedat a receiver.

According to one embodiment of the invention, there is provided anoptical signal with light occupying at least one wavelength, where thelight on each wavelength has an intensity that is pulse modulated by arespective optical data signal and amplitude modulated by a respectiveoverhead signal. The overhead signal has an RMS value that is a functionof an average intensity of the respective optical data signal andfurther having an instantaneous frequency content that is time-varying.The invention further provides that the instantaneous frequency contentof each overhead signal is uniquely associated with the respectivewavelength and also that the instantaneous frequency content of eachoverhead signal is harmonically related to a common fundamentalfrequency.

The number of wavelengths is usually at least two and is much larger ina dense system. The RMS value of each overhead signal may beproportional to the average intensity of the respective optical datasignal. Variations of the frequency content of at least one overheadsignal may encode control information. The combined frequency content ofeach overhead signal may span a frequency range between 300 kilohertzand 1 megahertz.

According to another embodiment of the invention, there are provided amethod and system for generating a dithered optical signal, includingthe steps of intensity modulating an optical carrier modulated datasignal in accordance with an overhead signal to produce the ditheredoptical signal, measuring a modulation depth of the dithered opticalsignal and adjusting the amplitude of the overhead signal as a functionof the measured modulation depth. The overhead signal has aninstantaneous frequency content which is time-varying and is uniquelyassociated with the optical wavelength of the carrier, wherein theinstantaneous frequency content is selected from a set of frequencieswhich are harmonically related to a common fundamental frequency.

According to still another embodiment of the invention, there isprovided a method of generating first and second dithered opticalsignals, each of which is generated according to the above describedmethod. In this embodiment, it is required that the frequency content ofthe first and second overhead signals be harmonically related to acommon fundamental frequency. The first dithered optical signal may beoptically coupled to the second dithered optical signal.

According to yet another embodiment of the invention, there are provideda method and system for determining the average intensity of at leastone optical data signal generated in a received optical signal havingbeen according to any method which ensures that each optical data signaloccupies a respective wavelength of interest and has an intensity thatis amplitude modulated by a respective overhead signal having aninstantaneous frequency content that varies with time.

In this embodiment, the method includes the steps of transforming thereceived optical signal into an aggregate signal in electrical form,transforming the aggregate signal into a frequency domain vector and,for each wavelength of interest:

-   -   (a) correlating the frequency-domain vector with a plurality of        harmonically related frequency-domain templates to produce        plural correlation results, the templates being uniquely        associated with the wavelength of interest;    -   (b) processing the plural correlation results to produce a        candidate frequency and a candidate RMS value; and    -   (c) validating the candidate frequency and the candidate RMS        value.

The step of transforming the aggregate signal into a frequency-domainvector may include sampling the aggregate electrical signal andexecuting a fast Fourier transform (FFT) on a finite number of samples,to produce the frequency-domain vector. The method may include theintermediate steps of acquiring the samples over an integer number ofblocks each having a length substantially equal to the inverse of thefundamental frequency, summing each block of windowed samples on anelement-by-element basis to produce a block of summed elements anddividing each summed element by the integer number.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects and features of the present invention will nowbecome apparent to those of ordinary skill in the art upon review of theaccompanying description of specific embodiments of the invention inconjunction with the following drawings, in which:

FIG. 1 is a time-domain, per-wavelength depiction of a waveform thatcarries control information in accordance with a specific embodiment ofthe present invention;

FIG. 2 is a frequency plan for multiple orthogonal frequency-shift-keyed(FSK) overhead signals associated with respective optical wavelengths,according to a specific embodiment of the present invention;

FIG. 3 is a block diagram of a system used for generating a singledithered optical signal in accordance with a specific embodiment of theinvention;

FIG. 4 is a block diagram of a system used for processing amulti-wavelength signal containing multiple dithered optical signals;and

FIG. 5 is a functional block diagram illustrating operation of the basicprocessing unit in FIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference is first made to FIG. 1 which shows a plurality of ditheredoptical signals 10, 20, 30 that are carried on respective distinctwavelengths by a common optical transmission medium (e.g., an opticalfiber or silica waveguide). In an actual DWDM network, there may be upto several hundred such dithered optical signals sharing the sameoptical fiber. Each of the dithered optical signals 10, 20, 30 consistsof light which has been twice modulated, once by a stream of data pulsesand another time by an overhead signal.

Specifically, each of the dithered optical signals 10, 20, 30 is made upof a respective one of a plurality of high-speed (e.g., 10 Gbps) opticaldata signals 12, 22, 32, each of which is typically binary pulsemodulated light that represents a logic zero when it has a low intensityand a logic one when it has a high intensity. The desired intensity ofthe light at the low intensity is typically the same for all the opticaldata signals 12, 22, 32 while the desired intensity of the light at thehigh intensity will be different, depending on the optical power withwhich the respective optical data signal is transmitted.

Each of the optical data signals 12, 22, 32 is further amplitudemodulated at its source by a respective one of a plurality oflow-frequency overhead signals 14, 24, 34. The amplitude of eachoverhead signal is chosen such that the modulation depth (i.e., theratio of the RMS value of the overhead signal to the average intensityof the corresponding optical data signal) is equal to a known value.This allows an eventual downstream network element to obtain,inferentially, the value of the average intensity of each optical datasignal simply by measuring the RMS value of the corresponding overheadsignal and multiplying it by the known modulation depth.

According to the invention, each of the overhead signals 14, 24, 34offers frequency diversity in order to provide immunity against, forinstance, framing of the optical data signals at 8 kHz. To this end, thecontrol channel employs frequency shift keying (FSK) having a relativelysmall number of frequency possibilities, which also keeps the receivercomplexity at a relatively low level.

The frequency of each overhead signal can thus be changed on a regularbasis to provide frequency diversity. Alternatively, the frequency ofeach overhead signal can be changed in accordance with controlinformation, thereby causing the overhead signal to serve as a low bitrate “control channel”. The control information could includeconnectivity information, such as the address of the source transmitterand/or the bit rate (e.g., OC-48, OC-192) and/or format (e.g., SONET,SDH) of the corresponding optical data signal. Further examples ofuseful control information may be found in co-pending U.S. patentapplication Ser. No. 09/199,327 of Harley et al., filed Nov. 25, 1998,assigned to the assignee of the present invention and incorporated byreference herein.

Thus far, the bit rate of each control channel, the number of frequencypossibilities for each overhead signal and possible values for thefrequencies themselves have not been discussed. Regarding the bit rateof each control channel and the number of frequency possibilities foreach overhead signal, these should be sufficiently low in order to avoidinterference of a particular overhead signal with the correspondingoptical data signal and also to avoid causing wild excursions of theresulting dithered optical signal.

One example of a suitable bit rate for the control channel at a givenwavelength is 2 bits per second and one example of a suitable number offrequency possibilities is 8. Given these two parameters, the minimalinterval between changes to the frequency content of the overhead signalwill be 1.5 seconds. Of course, other bit rates and other numbers offrequency possibilities are possible.

The issue of possible frequency values is now discussed. Firstly, itshould be recognized that although the various overhead signals 14, 24,34 occupy different wavelengths, the corresponding multiple ditheredoptical signals 10, 20, 30 travel together on the same fiber andtherefore the overhead signals will all be combined into a singleaggregate overhead signal if a single optical tap coupler is used at adownstream receiver.

In order to prevent the overhead signals 14, 24, 34 from interferingwith one another when they are combined into the aggregate overheadsignal, it is necessary to ensure their orthogonality. This can beachieved by assigning a unique set of frequencies to each overheadsignal, with each set being uniquely associated with the wavelength ofthe corresponding optical data signal. Thus, each overhead signal isonly allowed to occupy frequencies from an assigned set.

Next, it should also be recognized that a mere random assignment ofdistinct frequencies for each set will result in prohibitively complexreceiver equipment, insofar as sampling the overhead signals, decodingthe connectivity information and estimating the average opticalintensity at each wavelength are concerned. To achieve advantagesrelated to receiver simplification, the present invention requires thatall the frequencies used for all the overhead signals be harmonicallyrelated to a common fundamental frequency (denoted f₀), although f₀ neednot itself be a member of the set of frequencies associated with anyparticular wavelength.

The choice of f₀ is discussed in more detail at various points in thesequel and is basically influenced by the number of wavelengths sharingthe same fiber, by the number of frequencies in each set of assignedfrequencies, by what is considered to be a convenient FFT size (e.g., apower of 2), and by the range of possible sampling rates at thereceiver.

By way of example, a suitable frequency plan for 256 wavelength channelsis shown in FIG. 2, where there are taken to be 256 different frequencysets 1001, 1002, . . . , 1256, each containing 8 individual frequenciesA, B, C, D, E, F, G, H. The frequencies in each set could be evenlyspaced as shown, although they are only actually required to be spacedby integer multiples of f₀.

As shown, the sets of frequencies 1001, 1002, . . . , 1256 can becontiguous and each adjacent pair of frequency sets is separated by aguard band 50. In order to preserve the harmonic relationshiprequirement of the frequencies in each set, the guard bands 50 shouldeach have a width that is a multiple of the fundamental frequency f₀. Inthis particular case, the width of all guard bands 50 has been chosen tobe equal to twice the fundamental frequency f₀. Hence, in this example,the lowest frequencies of any two adjacent channels are separated bynine times the fundamental frequency f₀. It should be understood thatguard bands are not required, in which case the sets of frequencies neednot be contiguous (e.g., they may be interleaved).

In the example of FIG. 2, the fundamental frequency f₀ has been chosenas 32.2265625 Hz, the first set of frequencies begins at 318.75 kHz, andthe last set of frequencies ends at 392.94 kHz. In this particularexample, therefore, the total frequency span is equal to 74.19 kHz. Ingeneral, the lowest frequency (A) in the lowest set of frequencies(1001) should be sufficiently high to avoid small signal losses duecascaded erbium-doped fiber amplifiers (EDFAs), while the highestfrequency (H) in the highest set of frequencies (1256) should besufficiently low to avoid complexity issues at the receiver side. Tomeet these requirements, a suitable total frequency span will desirablyoccupy a subset of the 300 kHz to 1 MHz range, although other ranges arepossible.

Because of the required harmonic relationship of the frequencies in eachset to the fundamental frequency f₀, selection of the total frequencyspan and the number of frequency sets has a direct impact on thefundamental frequency f₀ and vice versa. Ideally, for reasons ofspectral efficiency, the fundamental frequency f₀ should be low enoughsuch that the corresponding total frequency span is in a desirablerange, e.g., 300 kHz to 1 MHz. The actual choice of f₀ within theseboundaries will affect the sampling rate (denoted f_(R)) which must beused at a receiver and will therefore depend on receiver requirements asdescribed in further detail later on.

Generation and detection of dithered optical signals is now described.Those skilled in the art will appreciate that multiple dithered opticalsignals sharing the same optical fiber are usually generated atdifferent source points in the optical network. Therefore, there existthe separate problems of (a) generating a single dithered optical signalat a source network element and (b) processing multiple dithered opticalsignals at a downstream (intermediate) network element. A system forgeneration of a single dithered optical signal will now be describedwith reference to FIG. 3 and a system for processing multiple ditheredoptical signals will be described later on with reference to FIGS. 4 and5.

With reference to FIG. 3, therefore, there is shown a system forgenerating a dithered optical signal (denoted O_(D)) on a tributaryoptical fiber 306 and for coupling, at a coupler 304, the ditheredoptical signal O_(D) to a main optical signal arriving on an inputoptical fiber 302. The dithered optical signal O_(D) occupies awavelength of interest, while the main optical signal on the inputoptical fiber 302 consists of optical signal content on zero or morewavelengths with no optical signal content on the wavelength ofinterest.

The dithered optical signal O_(D) on the tributary optical fiber 306 isgenerated by a system comprising a variable optical modulator (VOM) 310,an optical data signal generator 312 and a control module 314.Specifically, the dithered optical signal O_(D) is generated by thevariable optical modulator 310, which modulates an optical data signal(denoted O_(T)) received from the optical data signal generator 312 as afunction of an electrical overhead signal (denoted E_(OH)) received fromthe control module 314.

The system of FIG. 3 could be located at a network element such as aswitch or add/drop mux/demux (ADM). The optical data signal generator312 generically represents a source of optical data, which may originatelocally or which may have arrived along an arbitrary direction fromanother network element.

According to a specific embodiment of the invention, the electricaloverhead signal E_(OH) produced by the control module 314 causes thedithered optical signal O_(D) produced by the VOM 310 to have thecharacteristics previously described with reference to FIGS. 1 and 2.That is to say, the resulting dithered optical signal O_(D) has amodulation depth that is related to the average intensity of the opticaldata signal O_(T), while the overhead signal E_(OH) itself has frequencyvariations that provide frequency diversity. One way of providingfrequency diversity is to encode connectivity information into thefrequency variations of the overhead signal E_(OH).

Also according to a specific embodiment of the invention, the possiblefrequencies of the overhead signal E_(OH) are uniquely associated withthe wavelength of the optical data signal O_(T) so that knowledge of thewavelength of the dithered optical signal carrying a particular overheadsignal will not be required when detecting and decoding that overheadsignal at a receiver located further downstream.

In order for the dithered optical signal O_(D) to possess these desiredcharacteristics, the control module 314 desirably utilizes an FSK signalgenerator 210 or some similarly suitable generation means for generatingan electrical control signal (denoted E_(C)) which exhibits frequencyvariations. These frequency variations may encode connectivityinformation. In either case, the frequency of the control signal E_(C)at any given instant belongs to a set of frequencies that is uniquelyassociated with the wavelength of the optical data signal O_(T).

The FSK signal generator 210 is connected to a controllable gainamplifier 220, which generates the overhead signal E_(OH) by amplifyingthe control signal E_(C) in accordance with an electrical gain controlsignal (denoted E_(G)) received from a dither processing module 230.

The value of the gain control signal E_(G) is controlled by the ditherprocessing module 230 so that the resulting overhead signal E_(OH)produced by the amplifier 220 is at the appropriate amplitude, resultingin a known (or “reference”) modulation depth (denoted M_(D)) of thedithered optical signal O_(D) at the output of the VOM 310.

To this end, the dither processing module 230 is made up of a feedbackpath 240, a detection unit 250 and a comparison unit 260. The feedbackpath 240 is connected between the output of the VOM 310 and an input ofthe detection unit 250. The feedback path 240 comprises an optical tapcoupler 242, a photodiode 244 and a trans-impedance amplifier 246. Theoptical tap coupler 242 taps a portion of the optical power of thedithered optical signal O_(D). The photodiode 244 converts the tappedoptical signal to a photocurrent which is then converted to a voltage bythe trans-impedance amplifier 246.

The trans-impedance amplifier 246 thus supplies an input of thedetection unit 250 with a feedback signal (denoted E_(F)), which is anelectrical version of the dithered optical signal O_(D). Thus, thefeedback signal E_(F) has an overhead portion which closely matches thelow rate FSK overhead signal E_(OH). However, the remainder of thefeedback signal E_(F) does not necessarily accurately represent thevariations in the optical data signal O_(T) because the coupler 242,photodiode 244 and/or the trans-impedance amplifier 246 may bebandwidth-limited components. Nevertheless, it is usually the case thatthe feedback signal E_(F) does have a DC voltage which is representativeof the average intensity of the optical data signal O_(T).

The detection unit 250 comprises circuitry, control logic or softwarefor (a) determining the RMS value of the overhead portion of thefeedback signal E_(F); (b) determining the DC voltage of the feedbacksignal E_(F); and (c) computing the quotient of the two results in orderto obtain a modulation depth estimate (denoted M_(D′)).

In order to determine the RMS value of the overhead portion of thefeedback signal E_(F), the detection unit 250 performs a correlationbetween the feedback signal E_(F) and all possible frequencies of theelectrical control signal E_(C). To this end, the detection unit 250 maycomprise an analog-to-digital converter (ADC), a plurality oftime-domain correlators, a peak detector and various digital or analoglow-pass filters. The complexity requirements of the detection unit 250are relatively low because the feedback signal E_(F) contains a singleoverhead signal having a limited number of frequency possibilities.

Improved correlation accuracy and further reduced complexity can berealized through synchronous correlation of the feedback signal E_(F)with the control signal E_(C). This can be achieved by providing adelayed replica of the control signal E_(C) from the output of the FSKsignal generator 210 via a delay element 255. If used, the delay element255 should introduce a delay that is approximately equal to the delaythrough the amplifier 220, the VOM 310, the splitter 242, the photodiode244 and the trans-impedance amplifier 246.

In order to determine the DC voltage of the feedback signal E_(F), thedetection unit 250 may comprise a simple capacitor orresistive-capacitive network. In order to determine the quotient of thetwo RMS value of the dither signal and the DC voltage of the feedbacksignal, thereby obtaining the modulation depth estimate M_(D′), thedetection unit 250 may comprise a digital division module or othersuitable circuitry, software and/or control logic.

The output of the detection unit 250 is thus the modulation depthestimate M_(D′) which is fed to the comparison unit 260. The comparisonunit 260 comprises circuitry, control logic or software for comparingthe received modulation depth estimate M_(D′) to the referencemodulation depth M_(D). The output of the comparison unit 260 is scaledand used to modify the electrical gain control signal E_(G) which is fedto the amplifier 220.

In operation, the control signal E_(C) is generated by the FSK signalgenerator 210 and, by virtue of its frequency variations and content,the control signal E_(C) may encode connectivity information. Thecontrol signal E_(C) is amplified by the amplifier 220 and emerges asthe overhead signal E_(OH). The overhead signal E_(OH) then modulatesthe intensity of the optical data signal O_(T), resulting in thedithered optical signal O_(D).

The dithered optical signal O_(D) is tapped by the coupler 242 andsubsequently converted into the feedback signal E_(F) by the photodiode244 and the trans-impedance amplifier 246. The detector unit 250correlates the feedback signal E_(F) with a (delayed) version of thecontrol signal E_(C) in order to determine the RMS value of the overheadportion of the feedback signal E_(F). The DC voltage of the feedbacksignal E_(F) is obtained at a capacitor output and remains unaffected byvariations due to the overhead portion of the feedback signal E_(F).

The detector unit 250 divides the RMS value of the overhead portion ofthe feedback signal E_(F) by the average DC voltage of the feedbacksignal E_(F) to obtain the modulation depth estimate M_(D′). Thecomparison unit 260 then adjusts the gain control signal E_(G) as afunction of the difference between the modulation depth estimate M_(D)′and the reference modulation depth M_(D). (The modulation depth estimateM_(D′) may be different from the reference modulation depth M_(D) due totemperature fluctuations, for example.)

It should be appreciated that convergence of the feedback loop in thecontrol unit 314 is attained when the modulation depth estimate M_(D′)is substantially equal to the reference modulation depth M_(D). In thisway, it is ensured that the dithered optical signal O_(D) has beenmodulated at the appropriate modulation depth M_(D) by the overheadsignal E_(OH).

By virtue of being coupled by the coupler 304, the dithered opticalsignal O_(D) occupying the wavelength of interest shares the outputoptical fiber 308 with other optical signals on other wavelengths. It isenvisaged that these other signals may also be dithered optical signalswhich have been modulated by respective overhead signals that are alsoFSK signals. If this is indeed the case, then the FSK signals associatedwith different wavelengths should have the characteristics describedearlier with reference to FIGS. 1 and 2.

While traveling through a portion of the network, a multi-wavelengthoptical signal suffers wavelength-dependent losses introduced by theoptical medium itself and by various intervening cascades oferbium-doped fiber amplifiers. These losses affect the intensity of eachdithered optical signal to a degree which may depend on the wavelength.However, for a given dithered optical signal, the corresponding overheadsignal and the corresponding optical data signal are affected in thesame way. Therefore, the ratio of the RMS value of the overhead signalto the average intensity of the optical data signal remains constant. Inother words, the modulation depth of each dithered optical data signalremains unchanged, despite attenuation of the dithered optical signal.

Hence, by measuring the RMS value of each overhead signal, anintermediate network element can infer the average intensity of thecorresponding optical data signal. It may also be of interest for theintermediate network element to decode connectivity informationassociated with one or more optical wavelengths. It would be highlydesirable to perform these tasks without having to provide opticaldemultiplexing circuitry at the intermediate network element.Furthermore, it would be desirable to perform these tasks withreasonably low computational complexity.

To this end, FIG. 4 shows a system for estimating the average intensityof one or more optical data signals occupying respective wavelengths ofan incoming multi-wavelength optical signal arriving on an optical fiber400. The system may also be used for decoding connectivity informationassociated with each of these wavelengths.

It is assumed that each individual wavelength in the incomingmulti-wavelength optical signal is occupied by a dithered optical signalthat has been modulated with an FSK overhead signal using a system suchas that shown in FIG. 3. Also, it is assumed that the modulation schemeused for multiple wavelengths is orthogonal as previously described withreference to FIGS. 1 and 2, where the instantaneous frequency of eachoverhead signal belongs to a distinct set of frequencies and isharmonically related to a common fundamental frequency f₀.

The system in FIG. 4 comprises a data acquisition section 410 connectedto a processing section 450. The data acquisition section 410 comprisesan optical splitter 412 which intercepts the optical fiber 400 thatcarries the incoming multi-wavelength optical signal. The splitter 412is connected in series to other components in the data acquisitionsection 410, including a photodiode 414, a trans-impedance amplifier416, a low-pass filter 418, an analog-to-digital converter 420 and anaveraging unit 422.

The photodiode 414 and the trans-impedance amplifier 416 can be ofstandard design; these components provide the low-pass filter 418 withan electrical signal (denoted E_(A)(t)) that is representative of theaggregate light present on the optical fiber.

Because it is only necessary to detect and decode the individualoverhead signals (which have a low bandwidth in comparison with theoptical data signals), it is desirable to filter out high-speedvariations of the electrical signal E_(A)(t). This is achieved by thelow-pass filter 418, which can be of standard design. The low-passfilter 418 ensures that the electrical signal (denoted E_(L)(t)) at itsoutput contains just the aggregate of the low-frequency overhead signalspresent on the multiple wavelengths.

Because the overhead signals are desirably limited to between 300 kHzand 1 MHz, the low-pass filter 418 can accordingly be band limited tobelow 1 MHz. Those skilled in the art will appreciate that the low-passfilter 418 could also be a band-pass filter with a pass band in therange of 300 kHz to 1 MHz. In any event, the filter used should have atleast as wide a bandwidth as the aggregate overhead signal whilefiltering out variations due to high speed data.

The analog-to-digital converter (ADC) 420 can also be of standard designand comprises circuitry for sampling the signal E_(L)(t) received fromthe low-pass filter 418 at a sampling rate (denoted f_(R)) to produce asample stream (denoted S_(L)(n)) at its output. Because E_(L)(t) is aband limited signal, the sampling rate f_(R) can actually be below theso-called Nyquist rate. This technique is known as “bandpass sampling”and requires that:

-   -   (a) f_(R) is at least as high as, but desirably higher than,        twice the total frequency span of the signal E_(L)(t); and    -   (b) f_(R) is less than twice the minimum frequency of the signal        E_(L)(t).

In the above described example of FIG. 2, wherein the allowablefrequencies of the overhead signals spanned from 318.75 kHz to 392.94kHz and wherein f₀ was set to 32.2265625 Hz, requirements (a) and (b)restrict the value for the sampling rate f_(R) to between 148.38kilosamples per second (ksps) and 637.5 ksps.

In addition, it has already been mentioned that the choice of thefundamental frequency f₀ will dictate the choice of the sampling ratef_(R). In a specific sense, f_(R) should be chosen such that:

-   -   (c) 2^(N) samples are produced in T₀ seconds, for an integer N,        where T₀ is the inverse of the fundamental frequency f₀; and    -   (d) it is a multiple of the fundamental frequency f₀.

Requirement (c) facilitates execution of a subsequent fast Fouriertransform (FFT) in the processing section 450, while requirement (d)ensures that the frequency points of this ensuing FFT will fall exactlyat the frequencies chosen for the overhead signals. In the above exampleof FIG. 2, requirements (c) and (d) further restrict the sampling rateto either 528,000 samples per second (for an FFT size of 16,384) or264,000 samples per second (for an FFT size of 8192).

It should therefore be apparent that there is interplay between thesampling rate f_(R) and the fundamental frequency f₀. In particular, thechoice of the fundamental frequency f₀ directly impacts the choice ofthe sampling rate f_(R) (and vice versa), just as selection of thefundamental frequency f₀ had a direct impact on the choice of the totalfrequency span and the number of frequencies in each frequency set (andvice versa).

Considering now the averaging unit 422, this component comprisescircuitry, software or control logic for buffering the sample streamS_(L)(n) exiting the ADC 420 for the duration of an acquisition intervalof T_(acq) seconds. The number of samples buffered during thisacquisition interval of length T_(acq) seconds is denoted Z_(TOTAL) andis equal to Z₀×Y, where:

-   -   Z₀ is the number of samples arriving per fundamental period of        T₀ seconds (recalling that T₀=1/f₀); and    -   Y is an integer greater than or equal to 1, representing the        number of “blocks” per acquisition interval of T_(acq) seconds.

The averaging unit 422 also comprises circuitry, software and/or controllogic for performing a windowing operation on the set of Z_(TOTAL)samples buffered during T_(acq) seconds. In addition, the averaging unit422 also comprises suitable circuitry, software and/or control logic forseparating the windowed set of Z_(TOTAL) samples into Y blocks of Z₀samples and for averaging the blocks on an element-by-element basis.This produces an output block of Z₀ windowed and averaged samples,conveniently denoted in matrix form by [S_(A)(1) . . . S_(A)(Z₀)].

Thus, the output of the averaging unit 422 is a block of averagedsamples [S_(A)(1) . . . S_(A)(Z₀)], one such block being produced everyT_(acq) seconds. It should be apparent from the above that the elementsof this block of averaged samples [S_(A)(1) . . . S_(A)(Z₀)] are samplescorresponding to a windowed, periodically averaged version of theaggregate overhead signal formed from the individual overhead signalsassociated with the various wavelengths in the incoming multi-wavelengthoptical signal arriving on the optical fiber 400. It should also beapparent that Z₀ will be a power of two by virtue of having chosen thesampling rate f_(R) to result in the production of 2^(N) samples in T₀seconds for some integer N.

The averaging unit 422 feeds the block of averaged samples [S_(A)(1) . .. S_(A)(Z₀)] to a basic processing unit 452 located at the front of theprocessing section 450. In addition to comprising the basic processingunit 452, the processing section 450 also comprises a plurality (M) ofvalidation units 454 _(A)-454 _(M) connected to the basic processingunit 452. Connected to each validation unit is a respective one of aplurality of intensity estimators 456 _(A)-456 _(M) and a respective oneof a plurality of decoders 458 _(A)-458 _(M). M could equal 256 in aspecific embodiment consistent with the 256 frequency sets 1001-1256represented in FIG. 2.

The basic processing unit 452 comprises circuitry, software and/orcontrol logic for extracting and determining the frequency content ofeach individual overhead signal contained in the block of averagedsamples [S_(A)(1) . . . S_(A)(Z₀)]. Of particular interest are the RMSvalue and the instantaneous frequency of each overhead signal as derivedfrom the block of averaged samples [S_(A)(1) . . . S_(A)(Z₀)]. This isbecause each overhead signal consists of a tone whose instantaneousfrequency of the tone belongs to a restricted set of frequencies andchanges with time to provide frequency diversity and, possibly, toconvey control information. Also, each overhead signal (tone) has anassociated RMS value that is intended to be used by the receiver in FIG.4 for estimating the average optical intensity of the correspondingoptical data signal.

Now, it should be appreciated that the occurrence of a change in theinstantaneous frequency content of an overhead signal is unpredictablefrom the point of view of the processing section 450. In fact, it is notunlikely that during the last T_(acq) seconds in which an averagingoperation was performed to produce the Z₀ averaged samples [S_(A)(1) . .. S_(A)(Z₀)] arriving at the basic processing unit 452, there will havebeen an instantaneous frequency change for one or more of the overheadsignals. In such a “transitional” case, any attempt to estimate theinstantaneous frequency of an affected overhead signal, as well as theRMS value of that overhead signal at that frequency, will be flawed.

To resolve this issue (and as will be described in further detail lateron with reference to FIG. 5), the basic processing unit 452 produces,for each overhead signal associated with a respective opticalwavelength, a respective one of a plurality of “candidate” RMS values(denoted C_(RMS,A), . . . ,C_(RMS,M)) and a respective one of aplurality of “candidate” frequencies (denoted C_(FREQ,A), . . .,C_(FREQ,M)). For each overhead signal, these two respective outputs arefed along one or more control links to the respective one of thevalidation units 454 _(A)-454 _(M).

Each validation units 454 _(i), iε{A, B, . . . , M} comprises circuitry,software or control logic for processing the candidate RMS valueC_(RMS,i) and the candidate frequency C_(FREQ,i) associated with therespective overhead signal in order to determine whether a transitionhas occurred in the last T_(acq) seconds. If so, the validation unit inquestion labels the corresponding candidate frequency and its associatedRMS value as being invalid.

In one specific embodiment, the operation performed by each of thevalidation units 454 _(A)-454 _(M) may consist of determining whetherthe candidate RMS value associated with the respective overhead signalis above a certain pre-determined threshold. If not, it is concludedthat a transition is under way and that the candidate RMS value and thecorresponding candidate frequency are invalid. Of course, care should betaken to set the threshold to a value that is sufficiently low to allowfor a wide dynamic range of the overhead signal. In other words, 1 f thethreshold value is set too high, a particular validation unit mayconsistently conclude that the candidate RMS value is too low to bevalid, whereas the low value may in actuality be caused by severe,temporary, but possibly noncritical loss conditions on the fiber.

It is noted that because T_(acq) is much smaller than the time duringwhich the frequency content of an overhead signal remains constant(e.g., on the order of 1.5 seconds), an invalid version of the candidatefrequency and candidate RMS value will usually be followed by one ormore valid versions of the candidate frequency and candidate RMS value.When a valid candidate frequency and candidate RMS value has eventuallybeen found, the validation unit in question (say, 454 _(p)) will send a“valid RMS value” (denoted V_(RMS,p)) to its intensity estimator 456_(p) along one control line and a “valid frequency” (denoted V_(FREQ,p))to the decoder 458 _(p) along another control line.

Each of the intensity estimators 456 _(A)-456 _(M) comprises circuitry,software or control logic for dividing the valid RMS value of eachoverhead signal by the respective reference modulation depthM_(D,A)-M_(D,M) associated with that optical wavelength, thus arrivingat an estimate of the average intensity of the optical data signalcarried on the corresponding optical wavelength. The optical intensitydetermined in this manner can subsequently be processed by a controller(not shown), which may take action such as to increase the gain orinitiate protection switching if the computed intensity is found to beless than a pre-determined nominal value.

Regarding the decoders 458 _(A)-458 _(M), each such component comprisescircuitry, software and/or control logic for extracting the controlinformation, if any, encoded by the instantaneous frequency as well asthe frequency variations of the respective overhead signal. Suchinformation can include connectivity information and could be providedto a controller at the receiver, which may take switching action basedupon the decoded connectivity information.

Reference is once again made to the basic processing unit 452 in FIG. 4,operation of which is now described in greater detail with additionalreference to FIG. 5, wherein is shown a functional block diagramillustrating the operational steps executed by the basic processing unit452 when processing a block of Z₀ averaged samples [S_(A)(1) . . .S_(A)(Z₀)] received from the averaging unit 422 at intervals of T_(acq)seconds. It should be understood that the functional elements shown inFIG. 5 could be implemented in hardware, software, firmware or anycombination thereof.

At a fundamental level, the basic processing unit 452 shown in FIG. 5implements matched filter detection in the frequency domain which givessuperior performance in terms of noise removal. To this end, the firststep is the provision of a fast Fourier transform (FFT) functionalelement 510 at the input to the basic processing unit 452. The FFTfunctional element 510 performs an FFT of suitable size on the block ofZ₀ averaged samples [S_(A)(1) . . . S_(A)(Z₀)], received once everyT_(acq) seconds. The result of the FFT yields a spectrum vector denoted[X_(A)(1) . . . X_(A)(Z₀)], which shows the spectral content at each ofZ₀ evenly spaced frequencies.

The execution of a fast Fourier transform is simplified by virtue ofhaving made Z₀ a power of two through proper selection of the samplingrate f_(R) and the fundamental frequency f₀. In the specific examplesdescribed above, the fundamental frequency f₀ was chosen to equal32.2265625 Hz and a suitable sampling rate was found to be either528,000 samples per second (for Z₀=16,384) or 264,000 samples per second(for Z₀=8192).

It is at this point that the rationale behind requiring that a harmonicrelationship be maintained between the fundamental frequency f₀ and eachof the possible frequencies of each overhead signal as well as thesampling rate f_(R) becomes clear. In particular, it becomes clear thatby executing a single FFT (in this case a 16-K FFT or an 8-K FFT), thediscrete frequency points of the resulting spectrum vector [X_(A)(1) . .. X_(A)(Z₀)] will correspond exactly to the collection of possiblefrequencies for all the overhead signals.

Continuing with the functional description of the basic processing unit452, each of a plurality of correlators 520 _(i,j), iε{A, B, . . . , M},jε{A, B, . . . , N}, performs a frequency domain cross-correlationbetween the spectrum vector [X_(A)(1) . . . X_(A)(Z₀)] and a respectiveone of a plurality of spectral templates 525 _(i,j), iε{A, B, . . . ,M}, jε{A, B, . . . , N}. In this case, M is still the number of overheadsignals (i.e., the number of frequency sets in the spectrum), while N isthe number of frequencies per set. It should be appreciated that N maybe different for each overhead signal but is in this case assumed to beconstant in the interest of clarity. In the specific example describedin FIG. 2, M would equal 256 and N would equal 8.

Each of the spectral templates 525 _(i,j) can be a sparse vector havinga “one” at a single frequency point of interest and a “zero” elsewhere.Each correlator 520 _(i,j) may therefore perform an element-by-elementmultiplication of the spectrum vector [X_(A)(1) . . . X_(A)(Z₀)] withthe corresponding spectral template 525 _(i,j), to yield a sparse vectorhaving zeroes at all frequencies except for the frequency of thecorresponding spectral template, where the value of the sparse vector atthat frequency is the value of the spectrum vector [X_(A)(1) . . .X_(A)(Z₀)] at that frequency.

Next, a plurality of inverse FFT (IFFT) functional elements 530 _(i,j)performs short IFFTs on the output of the respective correlator 520_(i,j). To conserve computational resources, the length of each IFFT soperformed is desirably much shorter than the length of the FFT performedby the FFT functional element 510. Those skilled in the art will havelittle difficulty implementing a functional element which performs ashort IFFT on a longer input vector.

Because the input to each IFFT functional element 530 _(i,j) is a sparsevector with at most one non-zero element, the output of each IFFTfunctional element 530 consists of either zeroes or samples of a timedomain sinusoid (tone) at the template frequency. The result of the IFFTperformed by each of the IFFT functional elements 530 _(i,j) is thuspassed through a respective one of a plurality of peak detectors 540_(i,j) in order to determine the amplitude of the resulting time-domainsignal. It should be appreciated that during a change in theinstantaneous frequency of an overhead signal, two or more peakdetectors associated with that overhead signal may produce sinusoids ofcomparable amplitude.

Thus, the output of each of the peak detectors 540 _(i,j) for a commonoptical wavelength, i.e., for a common value of i, is fed to one of aplurality of maximum selectors 550 _(A)-550 _(M). Each of the maximumselectors 550 _(A)-550 _(M) is operable to identify the frequency of thetone which has the highest amplitude and to compute the RMS value ofthat tone. Since the relationship between RMS and peak values for asinusoid is well known, each of the maximum detectors 550 _(A)-550 _(M)may simply convert the output of the one peak detector generating thehighest peak value to an RMS value.

The frequency at which the maximum selector associated with the overheadsignal on a particular optical wavelength (say, maximum selector 550_(p)) has identified the maximum amplitude is output to thecorresponding validation unit 454 _(p) as the previously describedcandidate frequency C_(FREQ,p) for that overhead signal while the actualpeak value (converted to an RMS value) is provided to validation unit454 p as the previously described candidate RMS value C_(RMS,p). Each ofthe validation units 454 _(A)-454 _(M) then proceeds as previouslydescribed.

Other implementation options include using a different size of FFT,using a different size of IFFT, using different methods ofcross-correlation, each of which may be associated with its owntrade-off in terms of performance accuracy versus computationalcomplexity. Other options include simplifying the maximum detectionprocess by providing a simple software algorithm to run through theresults of the FFT functional element 510 and isolate, for each of the Msets of frequencies, the frequency point (and the corresponding FFTvalue) having the largest FFT value from among the frequency points inthe set.

Having considered the operation of the basic processing unit 452 withreference to FIG. 5, one of the main advantages of the invention shouldbecome clear. Specifically, because an FFT of length Z₀(=T₀×f_(R)) isperformed, this will produce points at frequencies that are spaced apartequally by f₀ hertz. Because of the required harmonic relationship amongall possible frequencies of all the overhead signals, the frequencies atwhich it is required to measure the RMS value fall exactly the frequencypoints provided by the FFT. Of course, small deviations which do notsubstantially affect the measurement accuracy are permissible.

Moreover, it should be appreciated that a high degree of accuracy in RMSestimation can be attained by performing a single FFT of manageable size(e.g., of length 16,384 or 8,192) every T_(acq) seconds. The complexityof the system is further kept in check since the IFFTs performed by theIFFT functional elements 530 _(i,j) are much shorter than the FFTperformed by the FFT functional element 510 _(i,j).

The invention also provides improved performance as a result of thespecially selected frequency range of the overhead signals, whichcontains frequencies sufficiently high to be immune to the small-signalresponse of cascaded EDFA amplifiers and sufficiently low to allow lowcomplexity at the intermediate network element where detection isrequired. Moreover, the fact that the overhead signals are FSK signalswith frequency content that is dynamic renders the overhead signalsimmune to periodic interference which may arise from the transmission offrame-based optical data signals.

Of course, there are other alternative configurations which belongwithin the scope of the invention. For example, it should be understoodthat the cross-correlation and IFFT operations performed subsequent tothe long 16K FFT could be executed in sequence rather than in parallel.Moreover, each overhead signal has been described as occupying only onefrequency at a time, but it is possible (although less energy efficient)to attain frequency diversity and convey control information by allowingan overhead signal to occupy more than one frequency at any giveninstant.

Furthermore, although the above description has dealt primarily withmulti-wavelength optical signals, those skilled in the art shouldappreciate that the invention can be applied to the generation andreception of an optical signal occupying a single carrier wavelength inthe optical spectrum. In such a case, there will be only one set offrequencies associated with the overhead signal (referring to FIG. 2)and the frequencies in that set will be harmonically related to a commonfundamental frequency in the above described manner.

The overhead signal will then occupy one (or possibly more) of thesefrequencies at a time and the actual instantaneous frequency will varywith time in order to provide frequency diversity. The time variationscould be used as a control channel to transmit control information.

The fact that frequency diversity is provided allows the system tocombat interference such as periodic interference due to framing of theunderlying optical data signal. The fact that the frequencies in the setare harmonically related allow the receiver system to achieve good RMSmeasurement accuracy at a relatively low computational complexitythrough a suitable choice of a sampling rate and FFT size, because eachpossible frequency in the overhead signal can be made to fall at thecenter of a frequency bin of the resulting FFT.

While specific embodiments of the present invention have been describedand illustrated, it will be apparent to those skilled in the art thatnumerous further modifications and variations can be made withoutdeparting from the scope of the invention as defined in the appendedclaims.

1. An optical signal, comprising: light occupying at least onewavelength, said light on each said wavelength having an intensity thatis pulse modulated by a respective optical data signal and amplitudemodulated by a respective overhead signal, said respective overheadsignal having an RMS value that is a function of an average intensity ofthe respective optical data signal and further having an instantaneousfrequency content that is time-varying; wherein the instantaneousfrequency content of each overhead signal is uniquely associated withthe respective wavelength and wherein the instantaneous frequencycontent of each overhead signal is harmonically related to a commonfundamental frequency.
 2. An optical signal as claimed in claim 1,wherein the RMS value of each overhead signal is proportional to theaverage intensity of the respective optical data signal.
 3. An opticalsignal as claimed in claim 1, wherein variations of the frequencycontent of at least one overhead signal encodes control information. 4.An optical signal as claimed in claim 2, wherein variations of thefrequency content of at least one overhead signal encodes controlinformation.
 5. An optical signal as claimed in claim 1, wherein thecombined frequency content of each overhead signal spans a frequencyrange from a lower frequency to a higher frequency where the lowerfrequency is above 300 kilohertz.
 6. An optical signal as claimed inclaim 1, wherein the combined frequency content of each overhead signalspans a frequency range from a lower frequency to a higher frequency andwherein the higher frequency is below 1 megahertz.
 7. An optical signalas claimed in claim 1, wherein the combined frequency content of eachoverhead signal spans a frequency range between 300 kilohertz and 1megahertz.
 8. An optical signal as claimed in claim 1, wherein thefrequency content of each overhead signal spans a respective frequencyrange and wherein the frequency range associated with any one overheadsignal does not overlap the frequency range associated with any otheroverhead signal.
 9. An optical signal as claimed in claim 8, wherein anyadjacent frequency ranges are separated by a guard band.
 10. An opticalsignal as claimed in claim 1, wherein the frequency content of eachoverhead signal spans a respective frequency range and wherein thefrequency range associated with each overhead signal overlaps thefrequency range associated with at least one other overhead signal. 11.An optical signal as claimed in claim 1, wherein each overhead signal isa frequency-shift keyed signal having an instantaneous frequencyselected from a finite set of frequencies.
 12. An optical signal asclaimed in claim 11, wherein the instantaneous frequency of any overheadsignal changes no faster than once every 1.5 seconds.
 13. An opticalsignal as claimed in claim 1, wherein the number of wavelengths is atleast two.
 14. An optical signal, comprising: light occupying at leasttwo wavelengths, said light on each of said wavelengths having anintensity that is pulse modulated by a respective optical data signaland amplitude modulated by a respective overhead signal, said respectiveoverhead signal having an RMS value that is a function of an averageintensity of the respective optical data signal and further having afrequency content that is time-varying; wherein the frequency content ofeach overhead signal is distinct from the frequency content of eachother overhead signal and wherein the frequency content of each overheadsignal is harmonically related to a common fundamental frequency.
 15. Anoptical signal as claimed in claim 14, wherein the RMS value of eachoverhead signal is proportional to the average intensity of therespective optical data signal.
 16. An optical signal as claimed inclaim 14, wherein variations of the frequency content of at least oneoverhead signal encodes control information.
 17. An optical signal asclaimed in claim 15, wherein variations of the frequency content of tleast one overhead signal encodes control information associated.
 18. Anoptical signal as claimed in claim 14, wherein the combined frequencycontent of all the overhead signals spans a frequency range from a lowerfrequency to a higher frequency where the lower frequency is above 300kilohertz.
 19. An optical signal as claimed in claim 14, wherein thecombined frequency content of all the overhead signals spans a frequencyrange from a lower frequency to a higher frequency and wherein thehigher frequency is below 1 megahertz.
 20. An optical signal as claimedin claim 14, wherein the combined frequency content of all the overheadsignals spans a frequency range between 300 kilohertz and 1 megahertz.21. An optical signal as claimed in claim 14, wherein the frequencycontent of each overhead signal spans a respective frequency range andwherein the frequency range associated with any overhead signal does notoverlap the frequency range associated with any other overhead signal.22. An optical signal as claimed in claim 21, wherein adjacent frequencyranges are separated by a guard band.
 23. An optical signal as claimedin claim 14, wherein the frequency content of each overhead signal spansa respective frequency range and wherein the frequency range associatedwith each overhead signal overlaps the frequency range associated withat least one other overhead signal.
 24. An optical signal as claimed inclaim 14, wherein each overhead signal is a frequency-shift keyed signalhaving an instantaneous frequency selected from a finite set offrequencies.
 25. An optical signal as claimed in claim 24, wherein theinstantaneous frequency of any one overhead signal changes no fasterthan once every 1.5 seconds.
 26. A method of determining the averageintensity of at least one optical data signal, each optical data signaloccupying a respective wavelength of interest in a received opticalsignal and having an intensity that is amplitude modulated by arespective overhead signal having an instantaneous frequency contentthat varies with time, the method comprising: (1) transforming thereceived optical signal into an aggregate signal in electrical form; (2)transforming the aggregate signal into a frequency domain vector; (3)for each wavelength of interest: (d) correlating the frequency-domainvector with a plurality of harmonically related frequency-domaintemplates to produce plural correlation results, said templates beinguniquely associated with the wavelength of interest; (e) processing theplural correlation results to produce a candidate frequency and acandidate RMS value; and (f) validating the candidate frequency and thecandidate RMS value.
 27. A method as claimed in claim 26, wherein thestep of transforming the aggregate signal into a frequency-domain vectorcomprises: sampling the aggregate electrical signal; and executing afast Fourier transform (FFT) on a finite number of samples, to producethe frequency-domain vector.
 28. A method as claimed in claim 27,further comprising, between the steps of sampling and executing an FFT:acquiring the samples over an integer number of blocks each having alength substantially equal to the inverse of the fundamental frequency;summing each block of windowed samples on an element-by-element basis toproduce a block of summed elements; and dividing each summed element bysaid integer number.
 29. A method as claimed in claim 28, furthercomprising windowing the acquired samples prior to the step of summing.30. A method as claimed in claim 26, wherein the step of processingcomprises, for each wavelength of interest, performing an inverse fastFourier transform (IFFT) on each correlation result, performing a peakdetection on the result of each IFFT and performing a maximum detectionon the set of peak detection results associated with the wavelength ofinterest.
 31. A method as claimed in claim 26, wherein the step ofvalidating comprises verifying whether the candidate RMS value at thecandidate frequency is above a pre-determined threshold.
 32. A method asclaimed in claim 26, further comprising estimating the average intensityof the optical data signal at each wavelength from the respectivevalidated candidate RMS value.
 33. A method as claimed in claim 32,wherein estimating comprises dividing the respective validated candidateRMS value by a respective reference modulation depth.
 34. A method asclaimed in claim 26, further comprising decoding control informationfrom the value of the validated candidate frequency.
 35. A method asclaimed in claim 26, further comprising decoding control informationfrom variations in the value of the validated candidate frequency.
 36. Asystem for determining the average intensity of at least one opticaldata signal, each optical data signal occupying a respective wavelengthof interest in a received optical signal and having an intensity that isamplitude modulated by a respective overhead signal having aninstantaneous frequency content that varies in time, the systemcomprising: an opto-electronic conversion unit, for converting thereceived optical signal into a time-domain electrical signal; afrequency domain transformation unit connected to the optoelectronicconversion unit, for transforming portions of the time-domain electricalsignal into respective frequency-domain blocks; for each wavelength, aplurality of matched filters connected to the FFT module, wherein eachmatched filter is adapted to correlate frequency-domain blocks receivedfrom the frequency domain transformation unit with a unique templaterepresentative of frequency content which is harmonically related to acommon fundamental frequency; for each wavelength, a candidate selectionunit connected to the matched filters associated with that wavelength,wherein each candidate selection unit is adapted to produce onecandidate frequency and one candidate RMS value; and for eachwavelength, a validation unit connected to the candidate selection unitsassociated with that wavelength, for determining whether the candidatefrequency and the candidate RMS value are valid.
 37. A system fordetermining the average intensity of at least one optical data signal,each optical data signal occupying a respective wavelength of interestin a received optical signal and having an intensity that is amplitudemodulated by a respective overhead signal having an instantaneousfrequency content that varies with time, the system comprising: meansfor transforming the received optical signal into an aggregate signal inelectrical form; means, connected to the transforming means, fortransforming the aggregate signal into a frequency domain vector; means,connected to the second transforming means, for correlating, for eachwavelength of interest, the frequency-domain vector with a plurality ofharmonically related frequency-domain templates to produce pluralcorrelation results, said templates being uniquely associated with thewavelength of interest; means, connected to the correlating means, forprocessing, for each wavelength of interest, the plural correlationresults to produce a candidate frequency and a candidate RMS value; andmeans, connected to the processing means, for validating, for eachwavelength of interest, the candidate frequency and the candidate RMSvalue.